The present invention relates to a process and to a unit for the production of a fluid enriched in oxygen by cryogenic distillation of a mixture comprising nitrogen, oxygen and argon.
It relates in particular to a process and to a unit for the separation of air by cryogenic distillation which makes possible the production of pure oxygen, that is to say oxygen comprising at least 95 mol % of oxygen, preferably at least 98 mol % of oxygen or even 99.5 mol % of oxygen.
When it is desired to prepare pure oxygen, the oxygen necessarily has to be separated from the argon. If the columns of the unit all operate at a pressure above 2 bar, distillation is difficult.
The production of pure argon requires a column having more than 100 theoretical plates.
Patent Application EP-A-0 540 900 discloses a process for the production of impure oxygen in which a portion of the impure argon comprising at least 90% of argon from a mixing column is mixed with the residual nitrogen from a simple column. The mixing column operates at the same low pressure as the low pressure column, up to 1.75 bara.
EP-A-0 384 213 has a low pressure column operating at between 1.5 and 10 bara but the argon column operates at a lower pressure.
U.S. Pat. No. 4,932,212 discloses the case in which the low pressure column and the argon column operate at pressures between 1 and 2 bar.
EP-A-0 518 491 discloses a process for the production of gaseous nitrogen under pressure and secondarily liquid nitrogen, liquid argon and liquid oxygen, in which process the low pressure column and the argon column operate at a substantially identical pressure above 2.5 bara. No gaseous argon flow is produced.
EP-A-0 952 415 discloses a unit comprising a double column and an argon column operating with an output below the optimum output.
One aim of the present invention is to increase the output of pure oxygen from an air separation unit.
Another aim of the invention is to provide an air separation unit which is particularly well suited to demands for large amounts of nitrogen under pressure (typically in the case of integration with a gas turbine of an IGCC).
According to one subject-matter of the invention, a process for the production of a flow enriched in oxygen in a cryogenic distillation unit is provided which comprises the stages of:
a) cooling a feed flow comprising oxygen, nitrogen and argon and introducing this flow into a distillation unit comprising an auxiliary column for separating a flow comprising at least argon and oxygen and at least two other columns;
b) separating this flow by cryogenic distillation in the unit, in order to form fluids enriched in oxygen and in nitrogen;
c) conveying the flow comprising at least argon and oxygen from one of the other columns to the auxiliary column, the auxiliary column operating substantially at the same pressure as the column from which originates the flow comprising at least argon and oxygen, this pressure being between 2 and 10 bar absolute;
d) withdrawing a flow enriched in oxygen from a column of the unit comprising at least 95 mol % of oxygen, optionally 98 mol % of oxygen;
e) withdrawing a flow enriched in argon from the auxiliary column;
characterized in that at least a portion of the flow enriched in argon is discharged to the atmosphere and/or is used to regenerate reversible exchangers or adsorbent beds and/or at least a portion of the flow enriched in argon is used as product, after being mixed with a gas enriched in nitrogen from the unit and/or another unit.
For example, the flow enriched in argon or the flow enriched in argon mixed with a gas enriched in nitrogen can be conveyed upstream of the pressure-reducing device of a gas turbine.
The flow enriched in argon can comprise between 10 and 95 mol % of argon (or between 40 and 95 mol % of argon), between 2 and 40 mol % of oxygen and between 2 and 40 mol % of nitrogen.
Optionally, all the flow enriched in argon is discharged to the atmosphere and/or is used to regenerate reversible exchangers or adsorbent beds and/or is mixed with a residual gas from the unit and/or another unit and/or conveyed upstream of the pressure-reducing device of a gas turbine.
In this case, there may all the same be production of argon, for example by withdrawing a flow richer in argon from the auxiliary column, which is the product.
The flow enriched in argon which is discharged to the atmosphere and/or is used to regenerate reversible exchangers or adsorbent beds and/or which is mixed with a gas enriched in nitrogen from the unit and/or another unit and/or which is conveyed upstream of the pressure-reducing device of a gas turbine can constitute between 0.3 and 2% of the air, preferably between 0.5 and 1% of the air. For this reason, it is preferable to mix the flow enriched in argon with a gas enriched in nitrogen comprising at least 90 mol % of nitrogen, for example originating from the low pressure column of a double column, and to use the mixture for regenerating reversible exchangers or adsorbent beds and/or to convey the mixture to a gas turbine and/or to reduce the mixture in pressure in a turbine. Thus, the mixture formed comprises less than 2 mol % of argon, preferably less than 1 mol % of argon.
The low pressure column can operate between 2 and 10 bara, preferably above 2.5 bara.
For example, the unit can comprise an auxiliary column for separation of a flow comprising at least argon and oxygen and two other columns, including a high pressure column and a low pressure column connected thermally to one another, the auxiliary column being fed from the low pressure column.
Alternatively, the unit can comprise an auxiliary column for separation of a flow comprising at least argon and oxygen and at least three other columns, including a high pressure column, an intermediate pressure column and a low pressure column connected thermally to one another, the auxiliary column being fed from the low pressure column or the intermediate pressure column.
According to another subject-matter of the invention, an integrated process for separation of air and the production of energy is provided, in which process a gas enriched in nitrogen is conveyed from the column preferably operating at the lowest pressure to the gas turbine, after an optional compression stage, and a fluid enriched in oxygen is optionally conveyed from a column of the unit to a gasifier.
According to another subject-matter of the invention, a unit for the production of oxygen by cryogenic distillation is provided which comprises:
a) an auxiliary column and at least two other columns;
b) means for conveying a flow comprising oxygen, nitrogen and argon to one of the other columns;
c) means for withdrawing a flow enriched in oxygen from one of the other columns;
d) means for withdrawing a flow comprising at least argon and oxygen from one of the other columns and means for conveying this flow as feed to the. auxiliary column;
e) means for withdrawing a fluid enriched in argon from the auxiliary column;
characterized in that the auxiliary column comprises between 1 and 99 theoretical plates and in that the unit comprises pressure-reducing turbine, means for conveying a gas from the column operating at the lowest pressure, apart from the auxiliary column, to the pressure-reducing turbine, these means not comprising compression means, and means for conveying at least a portion of the fluid enriched in argon to the atmosphere and/or means for conveying at least a portion of the fluid enriched in argon to reversible exchangers or adsorbent beds, in order to regenerate them, and/or means for mixing at least a portion of the fluid enriched in argon with a residual gas from the unit or another unit and/or means for conveying at least a portion of the fluid enriched in argon to a gas turbine.
Preferably, there are no pressure-reducing means between the column feeding the auxiliary column and the auxiliary column.
The auxiliary column optionally comprises between 30 and 40 theoretical plates.
Thus, with an auxiliary column operating at the same pressure as the low pressure column and preferably operating at a pressure above 2 bar, the separation of oxygen and argon in the collector of the low pressure column is facilitated. In this case, the fluid enriched in argon withdrawn from the auxiliary column is not necessarily a final product from the unit but can be used to cool the flows entering the columns or to provide cooling by reduction in pressure.